How to create abstract properties in python abstract classes

问题

In the following code, I create a base abstract class Base. I want all the classes that inherit from Base to provide the name property, so I made this property an @abstractmethod.

Then I created a subclass of Base, called Base_1, which is meant to supply some functionality, but still remain abstract. There is no name property in Base_1, but nevertheless python instatinates an object of that class without an error. How does one create abstract properties?

from abc import ABCMeta, abstractmethod
class Base(object):
    __metaclass__ = ABCMeta
    def __init__(self, strDirConfig):
        self.strDirConfig = strDirConfig

    @abstractmethod
    def _doStuff(self, signals):
        pass

    @property    
    @abstractmethod
    def name(self):
        #this property will be supplied by the inheriting classes
        #individually
        pass


class Base_1(Base):
    __metaclass__ = ABCMeta
    # this class does not provide the name property, should raise an error
    def __init__(self, strDirConfig):
        super(Base_1, self).__init__(strDirConfig)

    def _doStuff(self, signals):
        print 'Base_1 does stuff'


class C(Base_1):
    @property
    def name(self):
        return 'class C'


if __name__ == '__main__':
    b1 = Base_1('abc')

回答1:

Since Python 3.3 a bug was fixed meaning the property() decorator is now correctly identified as abstract when applied to an abstract method.

Note: Order matters, you have to use @property before @abstractmethod

Python 3.3+: (python docs):

class C(ABC):
    @property
    @abstractmethod
    def my_abstract_property(self):
        ...

Python 2: (python docs)

class C(ABC):
    @abstractproperty
    def my_abstract_property(self):
        ...

回答2:

Until Python 3.3, you cannot nest @abstractmethod and @property.

Use @abstractproperty to create abstract properties (docs).

from abc import ABCMeta, abstractmethod, abstractproperty

class Base(object):
    # ...
    @abstractproperty
    def name(self):
        pass

The code now raises the correct exception:

Traceback (most recent call last):
  File "foo.py", line 36, in 
    b1 = Base_1('abc')  
TypeError: Can't instantiate abstract class Base_1 with abstract methods name

回答3:

Based on James answer above

def compatibleabstractproperty(func):

    if sys.version_info > (3, 3):             
        return property(abstractmethod(func))
    else:
        return abstractproperty(func)

and use it as a decorator

@compatibleabstractproperty
def env(self):
    raise NotImplementedError()

来源：https://stackoverflow.com/questions/5960337/how-to-create-abstract-properties-in-python-abstract-classes

标签

python

properties

abstract-class

decorator